An LED power supply device as disclosed in JP Pub. No. 2011-258517 (hereinafter referred to as “Document 1”) has been so far proposed as a power supply device configured to operate light-emitting diodes (LEDs). The LED power supply device disclosed in Document 1 employs two kinds of LED arrays, color temperatures of which are different from each other, as a light source. The LED power supply device is configured to vary a color temperature of the light source (a color mixing light) by adjusting currents (electric currents) flowing through the two kinds of LED arrays.
The LED power supply device disclosed in Document 1 periodically turns the two LED arrays on and off so that the two LED arrays are alternately lit per period. In this LED power supply device, a color temperature of the light source (the color mixing light) is adjusted by varying a ratio of two time periods in which the two LED arrays are respectively lit during one period.
Considered is a control that allows current to flow through only one LED array while current is prohibited from flowing through another LED array at an upper or lower limit of a color temperature adjusting range. In this case, a large potential difference occurs between both cathode ends of the two LED arrays because both anode ends of the two LED arrays are connected with each other. That is, each of the two LED array is formed of LEDs connected in series, one end of each LED array is a cathode end and another end thereof is an anode end. It is therefore required that two conduction paths electrically connected to the two cathode ends of them be spaced out in order to ensure electrical insulation of a substrate on which the LED arrays and a power supply circuit are mounted. However, this causes the substrate (circuit substrate) to increase in size.